N2O Reduction by Gemmatimonas aurantiaca and Potential Involvement of Gemmatimonadetes Bacteria in N2O Reduction in Agricultural Soils.

Agricultural soil is the primary N2O sink limiting the emission of N2O gas into the atmosphere. Although Gemmatimonadetes bacteria are abundant in agricultural soils, limited information is currently available on N2O reduction by Gemmatimonadetes bacteria. Therefore, the effects of pH and temperature on N2O reduction activities and affinity constants for N2O reduction were examined by performing batch experiments using an isolate of Gemmatimonadetes bacteria, Gemmatimonas aurantiaca (NBRC100505T). G. aurantiaca reduced N2O at pH 5-9 and 4-50°C, with the highest activity being observed at pH 7 and 30°C. The affinity constant of G. aurantiaca cells for N2O was 4.4| |µM. The abundance and diversity of the Gemmatimonadetes 16S rRNA gene and nosZ encoding nitrous oxide reductase in agricultural soil samples were also investigated by quantitative PCR (qPCR) and amplicon sequencing ana-lyses. Four N2O-reducing agricultural soil samples were assessed, and the copy numbers of the Gemmatimonadetes 16S rRNA gene (clades G1 and G3), nosZ DNA, and nosZ mRNA were 8.62-9.65×108, 5.35-7.15×108, and 2.23-4.31×109 copies (g dry soil)-1, respectively. The abundance of the nosZ mRNA of Gemmatimonadetes bacteria and OTU91, OUT332, and OTU122 correlated with the N2O reduction rates of the soil samples tested, suggesting N2O reduction by Gemmatimonadetes bacteria. Gemmatimonadetes 16S rRNA gene reads affiliated with OTU4572 and OTU3759 were predominant among the soil samples examined, and these Gemmatimonadetes OTUs have been identified in various types of soil samples.

The link between nitrous oxide emissions, microbial community profile and function from three full-scale WWTPs.

Few attempts have been made in previous studies to link the microbial community structure and function with nitrous oxide (N2O) emissions at full-scale wastewater treatment plants (WWTPs). In this work, high-throughput sequencing and reverse transcriptase-qPCR (RT-qPCR) was applied to activated sludge samples from three WWTPs for two seasonal periods (winter and summer) and linked with the N2O emissions and wastewater characteristics. The total N2O emissions ranged from 7.2 to 937.0 g N-N2O/day, which corresponds to an emission factor of 0.001 to 0.280% of the influent NH4-N being emitted as N2O. Those emissions were related to the abundance of Nitrotoga, Candidatus Microthrix and Rhodobacter genera, which were favored by higher dissolved oxygen (DO) and nitrate (NO3-) concentrations in the activated sludge tanks. Furthermore, a relationship between the nirK gene expression and N2O emissions was verified. Detected N2O emission peaks were associated with different process events, related to aeration transition periods, that occurred during the regular operation of the plants, which could be potentially associated to increased emissions of the WWTP. The design of mitigation strategies, such as optimizing the aeration regime, is therefore important to avoid process events that lead to those N2O emissions peaks. Furthermore, this study also demonstrates the importance of assessing the gene expression of nosZ clade II, since its high abundance in WWTPs could be an important key to reduce the N2O emissions.

Structure of nitrogen-converting communities induced by hydraulic retention time and COD/N ratio in constantly aerated granular sludge reactors treating digester supernatant.

This study investigated how hydraulic retention time (HRT) and COD/N ratio affect nitrogen-converting consortia in constantly aerated granules treating high-ammonium digester supernatant. Three HRTs (10, 13, 19 h) were tested at COD/N ratios of 4.5 and 2.3. Denaturing gradient gel electrophoresis and relative real-time PCR were used to characterize the microbial communities. When changes in HRT and COD/N increased nitrogen loading, the ratio of the relative abundance of aerobic to anaerobic ammonium-oxidizers decreased. The COD/N ratio determined the species composition of the denitrifiers; however, Thiobacillus denitrificans, Pseudomonas denitrificans and Azoarcus sp. showed a high tolerance to the environmental conditions and occurred in the granules from all reactors. Denitrifier genera that support granule formation were identified, such as Pseudomonas, Shinella, and Flavobacterium. In aerated granules, nirK-possessing bacteria were more diverse than nirS-possessing bacteria. At a low COD/N ratio, N2O-reducer diversity increased because of the presence of bacteria known as aerobic denitrifiers.